1. Field of the Invention
The present invention relates to a current excitation type light emitting element in which a light emitting substance is interposed between a pair of electrodes, a light emitting device having such a light emitting element, and an electronic device.
2. Description of the Related Art
A light emitting element having features such as thin shape, lightweight, and rapid response is expected to be applied to flat panel displays of the next generation. In addition, it is said that a light emitting device in which light emitting elements are arranged in matrix is superior to a conventional liquid crystal display device in viewing angle and visibility.
A light emitting element is formed by interposing a light emitting substance containing layer between a pair of electrodes (an anode and a cathode), and it is said that emission mechanism thereof is as follows: when a voltage is applied between both electrodes, holes injected from an anode and electrons injected from a cathode are recombined in a light emitting layer in the light emitting substance containing layer, thereby forming a molecular exciton by recombination in an emission center, and energy is released when the molecular exciton returns to a ground state to emit light. By such a mechanism, such a light emitting element is referred to as a current excitation type light emitting element. A singlet excitation state and a triplet excitation state can be given as types of an excitation state formed by a light emitting substance. Light emission from a singlet excitation state is referred to as fluorescence and light emission from a triplet excitation state is referred to as phosphorescence.
There are problems such that these light emitting substances are easily deteriorated by a physical or chemical influence such as heat, moisture, and oxygen, and are easily deteriorated when the light emitting element is manufactured or driven because these light emitting substances are generally formed in a thin film state with 50 nm or less in a case of a single layer. The prevention of such deterioration of the light emitting element leads to improvement of yield and reliability of the light emitting element.
In a case where such a light emitting element is applied to a light emitting device, an electronic device, and the like, the reduction of power consumption is also given as one of the objects. It is important to reduce a drive voltage of a light emitting element in order to reduce power consumption.
In a current excitation type light emitting element, emission intensity is determined by an amount of flowing current. Therefore, it is necessary to flow a large amount of current at a low voltage in order to reduce a drive voltage.
Previously, as a method for reducing a drive voltage, an approach of providing a buffer layer between an electrode and a layer including an organic compound having a light emitting property, has been attempted. For example, it is known that a drive voltage can be reduced by providing a buffer layer which is made of polyaniline (PANI) doped with camphorsulfonic acid, between indium tin oxide (ITO) and a light emitting layer (for example, refer to Non-Patent Document 1: Y. Yang, et al. Applied Physics Letters, Vol. 64 (10), 1245-1247 (1994)). It is explained that this is because of the excellent carrier injecting property of PANI to the light emitting layer. Note that in the Non-Patent Document 1, PANI which is the buffer layer is also considered to be a part of an electrode.
However, as described in the Non-Patent Document 1, PANI has a problem that transmittance becomes lower when a film thickness becomes thick. Specifically, it is reported that, in a film thickness of about 250 nm, the transmittance is less than 70%. In other words, since the problem is with the transparency of the material itself which is used for the buffer layer, light that is generated within an element cannot be extracted efficiently.